Chapter 1 - WHY PLASTIC OPTICS?
The many reasons why product designers decide to useplastic optics essentially fall into two categories: relative
low cost and the opportunity to use unique element configuration.Plastic optics have a number of advantages over glass.Foremost of these are lower cost, higher impact resistance,lighter weight and more configuration possibilities
for simplifying system assembly. Configuration flexibilityis especially useful in systems that can use aspherical
lenses to simplify system design and reduce pats count,
weight and cost. Moreover, light transmittance is comparableto that of high-grade crown glasses. Finally, the plasticsthat can break generally do not splinter like glass. The
fragments are larger and tend to be more obtuse and lesshazardous.
The chief disadvantages of plastic optics are comparativeintolerance to severe temperature fluctuation in some
systems and low resistance to scratching. These disadvantages,however, are far outweighed by the advantages
plastic brings to the majority of optical applications. Althoughplastic has less temperature tolerance than glass,
most optical systems do not operate in ambient, temperaturesbeyond the thermal limits of plastic elements. For
that matter, glass optical systems often will not withstandmuch more physical abuse than their plastic counterpartsand still function properly.
The variety of available glass optical raw materials is
much greater than that for plastic. This abundance of glassoptions translates to greater design freedom because of thewide selection of dispersions and indices of refraction.
However, creative use of plastic aspherics often compensatesfor the narrower choice of materials.
Low Cost: The injection molding process, Fig. 1.1, is
ideal for producing large volumes of parts economically.Multicavity molds allow a low-cost manufacturing process
2 WHY PLASTIC OPTICS?
Fig 1.1 - Injection-molded optical elements are formed in
steel molds that contain machined cavities with surfaces polished
to an optical quality (top). The molten raw material is
forced under pressure into the temperature-controlled mold
(bottom). After cooling, the parts are removed from the gates
and runners and require no further finishing process.
to be combined with comparatively inexpensive raw materialsto create a powerful economic advantage for large
production volumes. By carefully sizing the mold for requiredproduction volume, the break-even cost, compared
to the glass alternative, can be surprisingly low, Fig. 1.2.
HANDBOOK OF PLASTIC OPTICS 3
Fi g. 1.2 - Multicavity injection molds can be sized for specific
production-volume requirements to optimize break-even
cost.
Integral Mounting: The molding process permits mountingand assembly features such as mounting brackets,
holes, slots and flanges to be integral with the optical element.The result is a single-piece design that eliminates
mounting hardware and simplifies assembly and alignment.Assembly costs often are more than that of the optic itself,
so the benefits of using imaginative configurations are obvious.Furthermore, multiple elements can be combined in
unique optical configurations such as transceiver lenses fortransmitting and receiving simultaneously. Figures 1.3,
1.4, 1.5, 1.6 and 1.7 demonstrate special designs incorporatingboth multiple elements and integral mounting.
Fig. 1.8 is a multielement lens with a special edge configurationthat aligns and centers the elements; this design is
economically beneficial for high production volume applications.
4 WHY PLASTIC OPTICS?
HANDBOOK OF PLASTIC OPTICS 5
FI G. 1.4 - Coaxial lens design, combining a dual lens function
with mounting pads, simultaneously sends and receives light.
FI G. 1.5 - Mounting flange of this dual lens also prevents dust
from entering the optical system.
6 WHY PLASTIC OPTICS?
Fig . 1.6 - Integral post allows condenser lenses to be swung
into the optical path of a dual-function projection system as required.Fig . 1.7 - Transceiver lens has alignment notches to facilitate
assembly. The off-axis optic focuses a laser beam in a product
code-reading system.
HANDBOOK OF PLASTIC OPTICS 7
Fig . 1.8 - This optical doublet has an integral flange that providesproper airspace and centering. The assembly can be
snapped, glued, ultrasonically welded or heat-staked, depending
on size, required tolerance and available tooling.
Lens arrays that are economically impractical in glass
are comparatively easy to manufacture in plastic, even
though the moldmaking process is usually complex and expensive.Fig. 1.9 and 1.10 are examples of such arrays.
Aspheres: Virtually all glass optic grinding and polishingequipment employs mechanisms that utilize mechanical
movements for contouring spherical surfaces. Traditionally,finishing the optical inserts of a mold for injection and
compression-molding has been performed with a similar
Fig . 1.9 - Lens array with 120 lens elements has integral
mounting pads and alignment pins for accurate positioning in a
card reader..
8 WHY PLASTIC OPTICS?
FIG . 1.10 - Micro-lens array is 0.2-in. long, 0.06-in. wide, and
contains 52 0.007-in. diameter lenses. Part of an automatic focusingdevice, the array has pads at the ends and alignment
posts for mounting an aperture. The bottom view shows the
part magnified 50 x.
HANDBOOK OF PLASTIC OPTICS 9
Fig . 1.11 - Aspheric curves often allow optical system correctionusing fewer elements.
process. Hence, most optics produced have been spherical.Aspheric lenses (nonspherical surfaces) are highly desiredby optical designers, Fig. 1.11.
Designs using aspheres often contain fewer elements
and cannot be configured in the same way if only sphericalsurfaces are used. The complex process of producing aprecise aspherical mold cavity surface is required onlyonce for each cavity. Consequently, the injection moldingprocess is an economical means for exploiting the advantagesof aspheres. Optical designers are using aspheres increasinglyto reduce costs or to obtain performance un-availableby any other means. Fig. 1.12, 1.13, and 1.14 are
examples of aspheric lens designs.
Repeatability: The molding process can yield high lens-to-lens repeatability, which is often a significant advantageof plastic over glass. This repeatability can reduce systemassembly and alignment time, with minimal impact on thesystem tolerance budget.
Where extremely critical tolerances are required, plasticoptic makers apply process control techniques for even
tighter regulation of process variables. Production tolerancesfor diameter and thickness are +O.OOl in. or less for
lenses up to 1 inch in diameter. In contrast, high-volume,low-cost glass has a +0.002-in. tolerance for thickness.
Lens-to-lens focal length variation is typically 1 to 2%, but
10 WHY PLASTIC OPTICS?
Fig . 1.12 - Aspheric corrector cancels spherical aberrationproduced by the primary mirror in a Schmidt optical systemused in a projection television set.
Fig . 1.13 - Using aspheres in condensing systems results inmore uniform light distribution. Three examples of aspheric
condensers are shown here.
HANDBOOK OF PLASTIC OPTICS 11
Fig. 1.14 - Aspheric lens arrays or “bubble” lenses are often
used to magnify light-emitting diodes. Aspheric surfaces are
necessary to optimize magnification and viewing angle.
can be held to 0.1% for higher quality lenses. This precisionis a significant advantage over glass.
Weight: For a given volume, optical glasses weigh approximately
2.3 to 4.9 times as much as plastic. In small
lens systems, this weight difference generally is not important.But where larger elements are used, the lighter
weight of plastic may be the overriding selection factor,
Fig. 1.15.
Breakage: Breakage, like scratching, is seldom a problemin glass. However, the use of glass generally is ruled
out in special applications requiring high impact resistance.For example, Fig. 1.16 and 1.17 illustrate military
applications where glass was unsatisfactory because of resistance-to-breakage requirements. Both products are
made of polycarbonate because of its high impact resistance.The facemask lens of the helicopter pilot’s helmet
was designed to withstand the impact of a 22-caliber bullet..
12 WHY PLASTIC OPTICS?
Fig . 1.15 - This 6-in. diameter lens is 1.5in. thick at the center.In plastic, it weighs 0.85 lb; a typical glass version would
weigh 2.2 lb.
Surface Texturing: To disperse light or create a contrastingappearance without using special decorative hardware,
designers occasionally specify a surface texture. This featurecan be applied by etching, controlled abrasion or
some other mechanical means. However, for mass-.
HANDBOOK OF PLASTIC OPTICS 13
Fig . 1.16 - This face mask is made of polycarbonate to withstandsevere impact tests.
Fig . 1.17 - Polycarbonate rocket nose lens must withstand
severe temperature and impact tests.
produced parts, surface texturing is more economicallyobtained by incorporating the feature into the mold cavitysurface. Fig. 1.18 and 1.19 demonstrate the use of this
technique to diffuse light and to create a light barrier.
14 WHY PLASTIC OPTICS?
Fig . 1.18 - Condensing lens has controlled etching for even
light diffusion.
Light Transmission: Light transmittance of quality opticalplastics covers a wide range. The transmittance of
acrylic is higher than most optical glasses throughout thevisible spectrum. Other optical plastics have a transmittancetypical of many optical glasses, which is slightly
lower than that of acrylic. This lower transmission is not afactor in most applications.
The transmittance of optical plastics is better than mostglasses in the ultraviolet and near infrared wavelengths.The ultraviolet end of the spectrum is usually restricted byabsorbing additives used to protect UV-sensitive materials.Spectral response can be tailored to specific requirementsby adjusting the composition of the additives. Fig.
1.20 and Fig. 1.21 show two parts with additives providinga specific spectral response.
In most cases the determining factor for using plastic
optics is cost. It is in the best interest of the supplier to beas frank about tradeoffs as possible, because profit results.
HANDBOOK OF PLASTIC OPTICS 15
Fig . 1.19 - Parabolic reflector has etched surfaces on the
side-walls for good paint adhesion and maximum stray-light
absorption.
Fig . 1.20 - Detector lens, using dyed material, restricts transmissionin the lower visible wavelengths, but allows maximum
transmission in the region where the detector is sensitive.
16 WHY PLASTIC OPTICS?
Fig . 1.21 - Colored windows are used to absorb background
light in illuminated display applications.
from providing parts - not tooling. Therefore, the supplieris motivated to use engineering and tooling time in a
manner that makes long-term sense to everyone involved.
A plastic lensmaker needs a good estimate of productionquantity requirements in order to bring the advantages ofplastic optic technology to the buyer. This factor will be
discussed in more detail because the balance between toolingcost and part cost is a critical requirement for realizing
minimum total cost.
Chapter 1 - WHY PLASTIC OPTICS?
The many reasons why product designers decide to useplastic optics essentially fall into two categories: relative
low cost and the opportunity to use unique element configuration.Plastic optics have a number of advantages over glass.Foremost of these are lower cost, higher impact resistance,lighter weight and more configuration possibilities
for simplifying system assembly. Configuration flexibilityis especially useful in systems that can use aspherical
lenses to simplify system design and reduce pats count,
weight and cost. Moreover, light transmittance is comparableto that of high-grade crown glasses. Finally, the plasticsthat can break generally do not splinter like glass. The
fragments are larger and tend to be more obtuse and lesshazardous.
The chief disadvantages of plastic optics are comparativeintolerance to severe temperature fluctuation in some
systems and low resistance to scratching. These disadvantages,however, are far outweighed by the advantages
plastic brings to the majority of optical applications. Althoughplastic has less temperature tolerance than glass,
most optical systems do not operate in ambient, temperaturesbeyond the thermal limits of plastic elements. For
that matter, glass optical systems often will not withstandmuch more physical abuse than their plastic counterpartsand still function properly.
The variety of available glass optical raw materials is
much greater than that for plastic. This abundance of glassoptions translates to greater design freedom because of thewide selection of dispersions and indices of refraction.
However, creative use of plastic aspherics often compensatesfor the narrower choice of materials.
Low Cost: The injection molding process, Fig. 1.1, is
ideal for producing large volumes of parts economically.Multicavity molds allow a low-cost manufacturing process
2 WHY PLASTIC OPTICS?
Fig 1.1 - Injection-molded optical elements are formed in
steel molds that contain machined cavities with surfaces polished
to an optical quality (top). The molten raw material is
forced under pressure into the temperature-controlled mold
(bottom). After cooling, the parts are removed from the gates
and runners and require no further finishing process.
to be combined with comparatively inexpensive raw materialsto create a powerful economic advantage for large
production volumes. By carefully sizing the mold for requiredproduction volume, the break-even cost, compared
to the glass alternative, can be surprisingly low, Fig. 1.2.
HANDBOOK OF PLASTIC OPTICS 3
Fi g. 1.2 - Multicavity injection molds can be sized for specific
production-volume requirements to optimize break-even
cost.
Integral Mounting: The molding process permits mountingand assembly features such as mounting brackets,
holes, slots and flanges to be integral with the optical element.The result is a single-piece design that eliminates
mounting hardware and simplifies assembly and alignment.Assembly costs often are more than that of the optic itself,
so the benefits of using imaginative configurations are obvious.Furthermore, multiple elements can be combined in
unique optical configurations such as transceiver lenses fortransmitting and receiving simultaneously. Figures 1.3,
1.4, 1.5, 1.6 and 1.7 demonstrate special designs incorporatingboth multiple elements and integral mounting.
Fig. 1.8 is a multielement lens with a special edge configurationthat aligns and centers the elements; this design is
economically beneficial for high production volume applications.
4 WHY PLASTIC OPTICS?
HANDBOOK OF PLASTIC OPTICS 5
FI G. 1.4 - Coaxial lens design, combining a dual lens function
with mounting pads, simultaneously sends and receives light.
FI G. 1.5 - Mounting flange of this dual lens also prevents dust
from entering the optical system.
6 WHY PLASTIC OPTICS?
Fig . 1.6 - Integral post allows condenser lenses to be swung
into the optical path of a dual-function projection system as required.Fig . 1.7 - Transceiver lens has alignment notches to facilitate
assembly. The off-axis optic focuses a laser beam in a product
code-reading system.
HANDBOOK OF PLASTIC OPTICS 7
Fig . 1.8 - This optical doublet has an integral flange that providesproper airspace and centering. The assembly can be
snapped, glued, ultrasonically welded or heat-staked, depending
on size, required tolerance and available tooling.
Lens arrays that are economically impractical in glass
are comparatively easy to manufacture in plastic, even
though the moldmaking process is usually complex and expensive.Fig. 1.9 and 1.10 are examples of such arrays.
Aspheres: Virtually all glass optic grinding and polishingequipment employs mechanisms that utilize mechanical
movements for contouring spherical surfaces. Traditionally,finishing the optical inserts of a mold for injection and
compression-molding has been performed with a similar
Fig . 1.9 - Lens array with 120 lens elements has integral
mounting pads and alignment pins for accurate positioning in a
card reader..
8 WHY PLASTIC OPTICS?
FIG . 1.10 - Micro-lens array is 0.2-in. long, 0.06-in. wide, and
contains 52 0.007-in. diameter lenses. Part of an automatic focusingdevice, the array has pads at the ends and alignment
posts for mounting an aperture. The bottom view shows the
part magnified 50 x.
HANDBOOK OF PLASTIC OPTICS 9
Fig . 1.11 - Aspheric curves often allow optical system correctionusing fewer elements.
process. Hence, most optics produced have been spherical.Aspheric lenses (nonspherical surfaces) are highly desiredby optical designers, Fig. 1.11.
Designs using aspheres often contain fewer elements
and cannot be configured in the same way if only sphericalsurfaces are used. The complex process of producing aprecise aspherical mold cavity surface is required onlyonce for each cavity. Consequently, the injection moldingprocess is an economical means for exploiting the advantagesof aspheres. Optical designers are using aspheres increasinglyto reduce costs or to obtain performance un-availableby any other means. Fig. 1.12, 1.13, and 1.14 are
examples of aspheric lens designs.
Repeatability: The molding process can yield high lens-to-lens repeatability, which is often a significant advantageof plastic over glass. This repeatability can reduce systemassembly and alignment time, with minimal impact on thesystem tolerance budget.
Where extremely critical tolerances are required, plasticoptic makers apply process control techniques for even
tighter regulation of process variables. Production tolerancesfor diameter and thickness are +O.OOl in. or less for
lenses up to 1 inch in diameter. In contrast, high-volume,low-cost glass has a +0.002-in. tolerance for thickness.
Lens-to-lens focal length variation is typically 1 to 2%, but
10 WHY PLASTIC OPTICS?
Fig . 1.12 - Aspheric corrector cancels spherical aberrationproduced by the primary mirror in a Schmidt optical systemused in a projection television set.
Fig . 1.13 - Using aspheres in condensing systems results inmore uniform light distribution. Three examples of aspheric
condensers are shown here.
HANDBOOK OF PLASTIC OPTICS 11
Fig. 1.14 - Aspheric lens arrays or “bubble” lenses are often
used to magnify light-emitting diodes. Aspheric surfaces are
necessary to optimize magnification and viewing angle.
can be held to 0.1% for higher quality lenses. This precisionis a significant advantage over glass.
Weight: For a given volume, optical glasses weigh approximately
2.3 to 4.9 times as much as plastic. In small
lens systems, this weight difference generally is not important.But where larger elements are used, the lighter
weight of plastic may be the overriding selection factor,
Fig. 1.15.
Breakage: Breakage, like scratching, is seldom a problemin glass. However, the use of glass generally is ruled
out in special applications requiring high impact resistance.For example, Fig. 1.16 and 1.17 illustrate military
applications where glass was unsatisfactory because of resistance-to-breakage requirements. Both products are
made of polycarbonate because of its high impact resistance.The facemask lens of the helicopter pilot’s helmet
was designed to withstand the impact of a 22-caliber bullet..
12 WHY PLASTIC OPTICS?
Fig . 1.15 - This 6-in. diameter lens is 1.5in. thick at the center.In plastic, it weighs 0.85 lb; a typical glass version would
weigh 2.2 lb.
Surface Texturing: To disperse light or create a contrastingappearance without using special decorative hardware,
designers occasionally specify a surface texture. This featurecan be applied by etching, controlled abrasion or
some other mechanical means. However, for mass-.
HANDBOOK OF PLASTIC OPTICS 13
Fig . 1.16 - This face mask is made of polycarbonate to withstandsevere impact tests.
Fig . 1.17 - Polycarbonate rocket nose lens must withstand
severe temperature and impact tests.
produced parts, surface texturing is more economicallyobtained by incorporating the feature into the mold cavitysurface. Fig. 1.18 and 1.19 demonstrate the use of this
technique to diffuse light and to create a light barrier.
14 WHY PLASTIC OPTICS?
Fig . 1.18 - Condensing lens has controlled etching for even
light diffusion.
Light Transmission: Light transmittance of quality opticalplastics covers a wide range. The transmittance of
acrylic is higher than most optical glasses throughout thevisible spectrum. Other optical plastics have a transmittancetypical of many optical glasses, which is slightly
lower than that of acrylic. This lower transmission is not afactor in most applications.
The transmittance of optical plastics is better than mostglasses in the ultraviolet and near infrared wavelengths.The ultraviolet end of the spectrum is usually restricted byabsorbing additives used to protect UV-sensitive materials.Spectral response can be tailored to specific requirementsby adjusting the composition of the additives. Fig.
1.20 and Fig. 1.21 show two parts with additives providinga specific spectral response.
In most cases the determining factor for using plastic
optics is cost. It is in the best interest of the supplier to beas frank about tradeoffs as possible, because profit results.
HANDBOOK OF PLASTIC OPTICS 15
Fig . 1.19 - Parabolic reflector has etched surfaces on the
side-walls for good paint adhesion and maximum stray-light
absorption.
Fig . 1.20 - Detector lens, using dyed material, restricts transmissionin the lower visible wavelengths, but allows maximum
transmission in the region where the detector is sensitive.
16 WHY PLASTIC OPTICS?
Fig . 1.21 - Colored windows are used to absorb background
light in illuminated display applications.
from providing parts - not tooling. Therefore, the supplieris motivated to use engineering and tooling time in a
manner that makes long-term sense to everyone involved.
A plastic lensmaker needs a good estimate of productionquantity requirements in order to bring the advantages ofplastic optic technology to the buyer. This factor will be
discussed in more detail because the balance between toolingcost and part cost is a critical requirement for realizing
minimum total cost.